There is known from the Japanese patent application JP-61-10958 a two-phase symmetric stepping motor having two rotation senses which is formed of two single-phase stator parts superposed onto one another and with an angular shift of 90.degree. relative to one another.
The two single phase stator parts are fastened to one another, but are located in different planes. They are magnetically insulated from one another while being each provided with an excitation winding. Furthermore, each stator part defines an opening for a permanent magnet rotor common to both of them, the openings being aligned on the axis of rotation of the latter.
In order to define a stable position of the rotor, the edge of each opening of such stator parts is provided with two positioning notches which are diametrally aligned along a direction separated by 45.degree. relative to the principal direction of the magnetic flux passing through the rotor and generated by excitation of the corresponding winding. In the absence of excitation, the stable position of the rotor is attained when the North-South axis of the permanent magnet of the rotor is perpendicular to the alignment axis of the notches.
Thanks to this construction, the two stator parts work under exactly the same conditions with the rotor, however, because of their 90.degree. shift each assures the rotation of the rotor in a sense which is individual to it. If one admits that the volume of the permanent magnet of the rotor traversed by each of the two magnetic fluxes is the same, the resulting motor couple is identical for both senses of rotation.
This known motor exhibits several disadvantages. Initially, the volume occupied by the permanent magnet of the rotor is only partially used since each magnetic circuit surrounds less than the half of this volume. This disadvantage is further aggravated by the fact that the magnetic circuits are necessarily spread apart from one another along the rotor axis in order to assure the magnetic insulation thereof. There results therefrom a comparatively substantial height which is above all inconvenient when the motor is employed in horological applications.
Another disadvantage resides in the fact that the openings of the magnetic circuits must be aligned with precision along the rotor axis which causes manufacturing difficulties so as to increase in an exaggerated manner the cost of the motor.
The invention has as its purpose to provide an electric motor of the type indicated hereinabove which does not exhibit these disadvantages.